Re: [PATCH v6 07/18] khwasan: add tag related helper functions

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> This commit adds a few helper functions, that are meant to be used to
> work with tags embedded in the top byte of kernel pointers: to set, to
> get or to reset (set to 0xff) the top byte.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  arch/arm64/mm/kasan_init.c |  2 ++
>  include/linux/kasan.h      | 29 +++++++++++++++++
>  mm/kasan/kasan.h           | 55 ++++++++++++++++++++++++++++++++
>  mm/kasan/khwasan.c         | 65 ++++++++++++++++++++++++++++++++++++++
>  4 files changed, 151 insertions(+)
>
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 7a31e8ccbad2..e7f37c0b7e14 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -250,6 +250,8 @@ void __init kasan_init(void)
>         memset(kasan_zero_page, KASAN_SHADOW_INIT, PAGE_SIZE);
>         cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
>
> +       khwasan_init();
> +
>         /* At this point kasan is fully initialized. Enable error messages */
>         init_task.kasan_depth = 0;
>         pr_info("KernelAddressSanitizer initialized\n");
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 1c31bb089154..1f852244e739 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -166,6 +166,35 @@ static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
>
>  #define KASAN_SHADOW_INIT 0xFF
>
> +void khwasan_init(void);
> +
> +void *khwasan_reset_tag(const void *addr);
> +
> +void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr);
> +void *khwasan_preset_slab_tag(struct kmem_cache *cache, unsigned int idx,
> +                                       const void *addr);
> +
> +#else /* CONFIG_KASAN_HW */
> +
> +static inline void khwasan_init(void) { }
> +
> +static inline void *khwasan_reset_tag(const void *addr)
> +{
> +       return (void *)addr;
> +}
> +
> +static inline void *khwasan_preset_slub_tag(struct kmem_cache *cache,
> +                                               const void *addr)
> +{
> +       return (void *)addr;
> +}
> +
> +static inline void *khwasan_preset_slab_tag(struct kmem_cache *cache,
> +                                       unsigned int idx, const void *addr)
> +{
> +       return (void *)addr;
> +}
> +
>  #endif /* CONFIG_KASAN_HW */
>
>  #endif /* LINUX_KASAN_H */
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 19b950eaccff..a7cc27d96608 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -8,6 +8,10 @@
>  #define KASAN_SHADOW_SCALE_SIZE (1UL << KASAN_SHADOW_SCALE_SHIFT)
>  #define KASAN_SHADOW_MASK       (KASAN_SHADOW_SCALE_SIZE - 1)
>
> +#define KHWASAN_TAG_KERNEL     0xFF /* native kernel pointers tag */
> +#define KHWASAN_TAG_INVALID    0xFE /* inaccessible memory tag */
> +#define KHWASAN_TAG_MAX                0xFD /* maximum value for random tags */
> +
>  #define KASAN_FREE_PAGE         0xFF  /* page was freed */
>  #define KASAN_PAGE_REDZONE      0xFE  /* redzone for kmalloc_large allocations */
>  #define KASAN_KMALLOC_REDZONE   0xFC  /* redzone inside slub object */
> @@ -126,6 +130,57 @@ static inline void quarantine_reduce(void) { }
>  static inline void quarantine_remove_cache(struct kmem_cache *cache) { }
>  #endif
>
> +#ifdef CONFIG_KASAN_HW
> +
> +#define KHWASAN_TAG_SHIFT 56
> +#define KHWASAN_TAG_MASK (0xFFUL << KHWASAN_TAG_SHIFT)
> +
> +u8 random_tag(void);
> +
> +static inline void *set_tag(const void *addr, u8 tag)
> +{
> +       u64 a = (u64)addr;
> +
> +       a &= ~KHWASAN_TAG_MASK;
> +       a |= ((u64)tag << KHWASAN_TAG_SHIFT);
> +
> +       return (void *)a;
> +}
> +
> +static inline u8 get_tag(const void *addr)
> +{
> +       return (u8)((u64)addr >> KHWASAN_TAG_SHIFT);
> +}
> +
> +static inline void *reset_tag(const void *addr)
> +{
> +       return set_tag(addr, KHWASAN_TAG_KERNEL);
> +}
> +
> +#else /* CONFIG_KASAN_HW */
> +
> +static inline u8 random_tag(void)
> +{
> +       return 0;
> +}
> +
> +static inline void *set_tag(const void *addr, u8 tag)
> +{
> +       return (void *)addr;
> +}
> +
> +static inline u8 get_tag(const void *addr)
> +{
> +       return 0;
> +}
> +
> +static inline void *reset_tag(const void *addr)
> +{
> +       return (void *)addr;
> +}
> +
> +#endif /* CONFIG_KASAN_HW */
> +
>  /*
>   * Exported functions for interfaces called from assembly or from generated
>   * code. Declarations here to avoid warning about missing declarations.
> diff --git a/mm/kasan/khwasan.c b/mm/kasan/khwasan.c
> index e2c3a7f7fd1f..9d91bf3c8246 100644
> --- a/mm/kasan/khwasan.c
> +++ b/mm/kasan/khwasan.c
> @@ -38,6 +38,71 @@
>  #include "kasan.h"
>  #include "../slab.h"
>
> +static DEFINE_PER_CPU(u32, prng_state);
> +
> +void khwasan_init(void)
> +{
> +       int cpu;
> +
> +       for_each_possible_cpu(cpu)
> +               per_cpu(prng_state, cpu) = get_random_u32();
> +}
> +
> +/*
> + * If a preemption happens between this_cpu_read and this_cpu_write, the only
> + * side effect is that we'll give a few allocated in different contexts objects
> + * the same tag. Since KHWASAN is meant to be used a probabilistic bug-detection
> + * debug feature, this doesn’t have significant negative impact.
> + *
> + * Ideally the tags use strong randomness to prevent any attempts to predict
> + * them during explicit exploit attempts. But strong randomness is expensive,
> + * and we did an intentional trade-off to use a PRNG. This non-atomic RMW
> + * sequence has in fact positive effect, since interrupts that randomly skew
> + * PRNG at unpredictable points do only good.
> + */
> +u8 random_tag(void)
> +{
> +       u32 state = this_cpu_read(prng_state);
> +
> +       state = 1664525 * state + 1013904223;
> +       this_cpu_write(prng_state, state);
> +
> +       return (u8)(state % (KHWASAN_TAG_MAX + 1));
> +}
> +
> +void *khwasan_reset_tag(const void *addr)
> +{
> +       return reset_tag(addr);
> +}
> +
> +void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr)

Can't we do this in the existing kasan_init_slab_obj() hook? It looks
like it should do exactly this -- allow any one-time initialization
for objects. We could extend it to accept index and return a new
pointer.
If that does not work for some reason, I would try to at least unify
the hook for slab/slub, e.g. pass idx=-1 from slub and then use
random_tag().
It also seems that we do preset tag for slab multiple times (from
slab_get_obj()). Using kasan_init_slab_obj() should resolve this too
(hopefully we don't call it multiple times).


> +{
> +       /*
> +        * Since it's desirable to only call object contructors ones during
> +        * slab allocation, we preassign tags to all such objects.
> +        * Also preassign tags for SLAB_TYPESAFE_BY_RCU slabs to avoid
> +        * use-after-free reports.
> +        */
> +       if (cache->ctor || cache->flags & SLAB_TYPESAFE_BY_RCU)
> +               return set_tag(addr, random_tag());
> +       return (void *)addr;
> +}
> +
> +void *khwasan_preset_slab_tag(struct kmem_cache *cache, unsigned int idx,
> +                               const void *addr)
> +{
> +       /*
> +        * See comment in khwasan_preset_slub_tag.
> +        * For SLAB allocator we can't preassign tags randomly since the
> +        * freelist is stored as an array of indexes instead of a linked
> +        * list. Assign tags based on objects indexes, so that objects that
> +        * are next to each other get different tags.
> +        */
> +       if (cache->ctor || cache->flags & SLAB_TYPESAFE_BY_RCU)
> +               return set_tag(addr, (u8)idx);
> +       return (void *)addr;
> +}
> +
>  void check_memory_region(unsigned long addr, size_t size, bool write,
>                                 unsigned long ret_ip)
>  {
> --
> 2.19.0.rc0.228.g281dcd1b4d0-goog
>